The ratio of amylose to amylopectin in starch significantly affects the characteristics and quality of its finished food products including their digestibility, water retention, and resistance to staling. Starches that are high in amylose, a linear polymer, tend to gel when cooked whereas those that are high in amylopectin, a branching polymer, tend to form viscous pastes. Because of their unique physical properties including their pasting properties, solubility, gelling capacity, gel strength, swelling power, and vicosity, low amylose/high amylopectin starches are often used in the food industry to improve the texture and mouth-feel of select food products as well as their freeze-thaw stability.
Amylose synthesis in a variety of plants, including wheat, is regulated for the most part by the enzyme granule-bound starch synthase (GBSSI), also known as waxy protein. The importance of this gene in starch synthesis has been well documented in naturally occurring varieties of GBSSI-deficient rice and corn, termed waxy mutants. Following the commercialization of waxy rice and waxy corn starches, there has been extensive interest by wheat breeders and the U.S. Department of Agriculture to develop waxy wheat lines for use in the food industry as well as other commercial applications. Whereas starch from most traditional wheat cultivars is approximately 24% amylose and 76% amylopectin, starch from full waxy wheat lines (i.e., carrying deletions of all three genes) is almost 100% amylopectin. Potential commercial uses of waxy wheat starch include its use as a sauce thickener, emulsifier, and shelf-life extender. When mixed with traditional wheat flour in bread dough, waxy wheat flour improves crumb texture, freshness, and softness and eliminates the need for shortenings, thereby reducing fat content, unhealthy trans-fatty acids, and cost. Blended with other regular flours, waxy wheat flour improves the texture and tenderness of pasta and noodles, including Japanese udon noodles. In addition to the food industry, high amylopectin starches are important to the paper industry for enhancing the strength and printing properties of paper products and to the adhesive industry as a component of glues and adhesives, especially those used on bottles.
Though breeding programs are underway to develop commercial varieties of waxy wheat, the polyploid nature of the wheat genome combined with homoeologous chromosome pairing has made the identification of waxy wheat mutants through traditional breeding methods difficult. The majority of wheat traded in commerce is Triticum aestivum or bread wheat. In this hexaploid, waxy is encoded by three homoeologues, Wx-7A, Wx-4A, and Wx-7D with the chromosomal locations 7AS, 4AL, and 7DS (Murai et al., Isolation and characterization of the three Waxy genes encoding the granule-bound starch synthase in hexaploid wheat. Gene 234: 71-79, 1999). In order to breed full waxy varieties using traditional breeding methods, knock-out mutations of all three homoeologues are required. Although several hundred lines of wheat have been identified that carry one or more mutations in the waxy genes Wx-4A and Wx-7A, only four deletion mutations of Wx-7D have been identified to date in over three thousand wheat lines that have been evaluated. One of these is in a Chinese landrace called Bai Huo, whose genetic heterogeneity makes it less suitable for traditional wheat breeding programs than modern elite cultivars. A cross between a double waxy null, Kanto 107, and the Bai Huo landrace was performed to create the first full waxy null line in wheat (Nakamura et al., Mol Gen Genet 248: 253-259, 1995). Despite the recent development of waxy breeding lines using this starting material, commercial varieties of waxy wheat are still not available, presumably due to the difficulty of removing undesirable agronomic traits from exotic germplasm. The paucity of Wx-7D deletion mutations has severely limited the development of commercial waxy wheat lines through traditional breeding.
With the availability of the genetic sequences of the Triticum aestivum waxy genes, transgenic technology could be used to modify the expression of targeted proteins like waxy rather than rely on traditional breeding programs for the development of waxy wheat cultivars which could take years. However, public acceptance of genetically modified plants, particularly with respect to plants used for food, is low. Therefore, it would be useful to have additional commercial varieties of full or partial waxy wheat that were not the result of genetic engineering. The availability of multiple allelic mutations within each waxy locus would also allow for the breeding of new, diverse waxy phenotypes showing a spectrum of functional characteristics.